Generic ground compactors are provided for compacting the substrate and are well known in the prior art, for example in the form of road rollers or, more specifically, tandem rollers with which a ground layer structure, and in particular an asphalt road including its foundation, can be compacted. Essential elements of a ground compactor, in particular a self-propelled ground compactor, include a machine frame, a drive engine, a driver's cab as well as front and rear ground compaction drums with respect to the working direction. To achieve compaction, the ground compactor is moved over the ground layer structure to be compacted in several steps, further compaction being achieved with each passage until a maximum compaction is reached. Upon reaching the maximum compaction, further compaction of the substrate is no longer necessary or is even detrimental since this will result in a loosening of the compacted ground layer structure or grain shattering. For this reason, it is helpful to know the current degree of compaction of the substrate. Moreover, an aspect of particular importance when employing ground compaction rollers is the formation of bow waves. Due to the drum sinking into the substrate to a greater or lesser extent, ground material is displaced in a circumferential direction toward the front and the rear of the drum. The amount of material accumulated in front of the drum can become so large that, due to the displacement resistance, it is no longer pushed ahead of the drum but is rolled over by the same. This leads to an unevenness in the road surface.
Besides static ground compactors, with which ground compaction is achieved exclusively by the weight of the ground compaction drum, dynamic ground compactors are known in the art with which, in order to increase the compaction performance, the ground compaction drum not only rotates about a rotation axis when rolling over the substrate but also performs an additional movement in order to affect a transfer of grains in the substrate into a layer with a higher density. A distinction is made here between vibrating and oscillating ground compaction drums. With oscillating ground compaction drums, the rotational movement of the ground compaction drum in the direction of rotation is superimposed by an oscillating movement, resulting in a shearing force being exerted by the ground compaction drum on the surface of the substrate. This oscillating movement is normally caused by two oscillating masses rotated inside the ground compaction drum. In the case of vibrating ground compaction drums, the ground compaction drum enters a vibrating state in which the ground compaction drum performs a vibrating movement, whereby the ground compaction drum exerts a contact force on the substrate, said contact force being directed towards the substrate and time-modulated due to the vibrating movement. Due to the time modulation of the contact force and the resulting time-variable deformation of the substrate, the change in the contact force that occurs with vibrating ground compaction drums can be utilized for determining the degree of stiffness of the substrate, as is known, for example, from DE 10 2010 052 713 A1, said degree of stiffness being a measure of the compaction in the substrate. Since static ground compactors lack such a time-modulated contact force, the known methods for determining a degree of stiffness for vibrating ground compaction equipment cannot be transferred to static ground compactors. Although time-variable thrust forces do occur in ground compactors with an oscillating ground compaction drum, these forces mainly depend on the friction conditions between the drum and the substrate and are consequently rather ill-suited for ascertaining ground stiffness.